1. Field of the Invention
This invention relates to a fixing member of a fixing assembly used in image-forming apparatus employing an image-forming process such as electrophotography or electrostatic recording. More particularly, this invention relates to a fixing member used in a heat fixing assembly by means of which an unfixed toner image formed and carried on a recording material (such as a transfer material, printing paper, photosensitive paper or electrostatic recording paper) by a transfer system or a direct system in an image-forming processing section is treated by heat fixing to form a fixed image. It also relates to a fixing assembly having such a fixing member and an image-forming apparatus having the fixing assembly.
2. Related Background Art
Conventionally, in fixing assemblies provided in image-forming apparatus employing an image-forming process such as electrophotography or electrostatic recording, heat fixing assemblies are widely used in which a recording material (hereinafter also xe2x80x9ctransfer materialxe2x80x9d) holding an unfixed image thereon is passed through a nip formed between a fixing roller and a pressure roller which are rotated in pressure contact with each other, to fix the toner image to the recording material. An example of an image-forming apparatus having a conventional heat fixing assembly is shown in FIG. 5.
In the apparatus shown in FIG. 5, a fixing roller 10 which is a fixing member having a heating element comprises a hollow mandrel made of aluminum and a heating element halogen lamp provided inside the mandrel. From the interior of the hollow mandrel, a recording material is heated at a temperature high enough to cause the toner (toner image) held thereon to melt by supplying electricity from a power source (not shown).
On the periphery of the hollow mandrel, a releasing layer comprised of a material such as a polytetrafluoroethylene copolymer (PTFE) or a perfluoroalkoxytetrafluoroethylene copolymer (PFA) is formed which has an excellent performance for releasability and resistance to heat. The releasing layer is formed on the hollow mandrel by covering its surface with a material formed in a tube, or by coating it with a material by electrostatic spraying, dip coating or the like.
Such a fixing roller, however, has had a problem that it may cause a phenomenon of electrostatic offset in which the toner held on the recording material is electrostatically transferred to the fixing roller, resulting in a low image quality level.
Because of triboelectric charging taking place between the recording material and the fixing roller or because of transfer electric charges accumulated on the recording material, an electric field through which the toner on the recording material is attracted to the fixing roller is produced, so that a part of the toner image is transferred onto the fixing roller. After the fixing roller is rotated once, the toner thus transferred is fixed to the recording material to become a ghost on the image. This is called xe2x80x9celectrostatic offsetxe2x80x9d.
The electrostatic offset is roughly grouped into two types, whole-area offset and release offset. The whole-area offset is a phenomenon where the recording material and the fixing member such as the fixing roller give and take electric charges mutually through triboelectric charging to cause an offset electric field steadily and the offset appears over the whole image area continuously. Meanwhile, the release offset is a phenomenon where the recording material hops at its rear end to come into strong contact with the fixing roller when the rear end of the recording material goes through the fixing assembly, so that it leaves potential history linearly on the fixing roller in its longitudinal direction, which potential causes an offset, and on the image the offset appears linearly in the principal scanning direction. Thus, the both are distinguishable.
To prevent such electrostatic offsets, in conventional apparatus the potential of the fixing roller is controlled at a constant value. Stated specifically, where a negatively chargeable toner is used, the fixing roller is subjected to antistatic treatment so as not to be positively charged, or is set electrically conductive and grounded so as to be made to have a potential of 0 V.
In an experiment, surface potential of the surface layer of such a fixing roller was measured with a surface potentiometer during paper feeding, to find that the surface layer stood charged only at tens of V even during paper feeding, thus an antistatic effect was confirmed.
Meanwhile, as image quality and process speed of electrophotographic apparatus have been made higher in recent years, a phenomenon called xe2x80x9csmeared image trailing edgesxe2x80x9d (reading xe2x80x9csmeared image-trailing-edgesxe2x80x9d; also xe2x80x9cbleeding imagesxe2x80x9d) has come to occur conspicuously, in which a part of horizontal-line images diffuses toward its rear end side to become broken when the horizontal-line images are fixed. The cause of such smeared image trailing edges is presumed to be chiefly the pressure ascribable to steam generated from the interior of paper. The phenomenon of smeared image trailing edges is detailed here with reference to FIG. 6.
As shown in FIG. 6, steam having spouted at a space between a fixing roller 10 and a recording material 21 flows in the direction of an arrow and is compressed between the fixing roller 10, the recording material 21 and a toner image 41 to disorder the toner image 41.
As one of countermeasures for the smeared image trailing edges, it is proposed to provide a means for applying a bias voltage to the fixing roller 10. FIG. 7 shows an example of construction provided with such a bias-applying means (or bias power source) 12 as an example of countermeasures for the smeared image trailing edges.
As shown in FIG. 7, a fixing roller 10 has a fixing roller mandrel 10a formed in a hollow roll, and a releasing layer 10b which covers the periphery of the fixing roller mandrel 10a. To the fixing roller mandrel 10a, a DC bias of about 500 to 1,000 V is applied from the bias power source 12. Here, a resistance element (not shown) of several M xcexa9 to tens of M xcexa9 is provided as safety resistance across the bias power source 12 and the fixing roller mandrel 10a. 
By the aid of an electric field generated by this bias, the toner image on the recording material 21 is electrostatically strongly held on the recording material 21 when the recording material 21 rushes into a fixing nip 31. In this state, the recording material 21 with the toner image is held between the fixing roller 10 and a pressure roller 11 and transported therethrough. Hence, the smeared image trailing edges can be prevented from occurring on the recording material 21 even when the flow of steam as stated above is produced.
In the construction where the bias-applying means is provided as a countermeasure for the smeared image trailing edges, it is important to produce an electric field across the toner image 41 and the back of the recording material 21 through the recording material 21, which electric field attracts the toner to the part of the recording material 21. Accordingly, in such construction, a transport roller (not shown) serving as a contact member which comes into contact with the recording material 21 is provided immediately behind the fixing nip 31, and this roller is set electrically conductive and grounded so that the voltage applied to the fixing roller 11 causes an electric current through the recording material 21 to produce the electric field across the toner image 41 and the back of the recording material 21.
Here, any too low resistance of the fixing roller 10 may cause an increase in the voltage allotted to the safety resistance to lower the voltage that contributes to the formation of the electric field across the fixing roller 10 and the back of the recording material 21, so that the effect of preventing the smeared image trailing edges may lower.
In the controlling of resistance values of the fixing roller 10 by dispersing carbon or a charge controlling agent therein as conventionally done, the resistance values of the fixing roller may greatly change depending on the viscosity of a coating solution used when the fixing roller 10 is manufactured, its pH value, the state of dispersion of carbon or charge control agent therein and its changes with time. Thus, it has not been easy to control the resistance values of the fixing roller 10 to a constant value.
Moreover, even where carbon is dispersed in the fixing roller 10 in a small quantity, the fixing roller 10 can not easily be made to have a low surface resistivity. Accordingly, in order to make its surface resistivity low enough to attain the effect of preventing the electrostatic offset, the carbon must be dispersed in a large quantity. However, dispersing the carbon in a large quantity may make the fixing roller 10 have a very low volume resistivity. Hence, any effect of preventing smeared image trailing edges can not be attained, and it has been difficult to ensure a sufficient image quality.
As disclosed in Japanese Patent Application Laid-open No. 2000-19879, a heat fixing roller is proposed in which the releasing layer is formed in a multilayer and a filler is dispersed in each layer. In such construction, the fixing roller can be made to have a proper volume resistivity but can not easily be made to have a low surface resistivity. Thus, there is room for further improvement in proofness to electrostatic offset (i.e., anti-offset properties). In addition, a filler having a low powder resistivity may cause a great change in volume resistivity, making it difficult to control volume resistivity.
Accordingly, it has been long awaited to provide a method by which the releasing layer at the surface of the fixing roller 10 is made only to have a low surface resistivity without being made to have a low volume resistivity.
An object of the present invention is to provide a fixing member having solved the above problems, a fixing assembly having such a fixing member and an image-forming apparatus having the fixing assembly.
Another object of the present invention is to provide a fixing member which can stably be made to have a low surface resistivity and has superior anti-offset properties and proofness to smeared image trailing edges, a fixing assembly having such a fixing member and an image-forming apparatus having the fixing assembly.
To achieve the above objects, the present invention provides a fixing member for use in a fixing assembly, wherein the fixing assembly has at least the fixing member and a pressure member coming into pressure contact with the fixing member to form a fixing nip, where a recording material holding an unfixed toner image thereon is passed through the fixing nip so as to fix the unfixed toner image to the recording material to form a fixed imaged on the recording material;
the fixing member comprising a conductive layer and a releasing layer formed on the conductive layer, wherein;
in the releasing layer, any one or both of medium-resistance particles and medium-resistance whiskers are dispersed; and
the releasing layer has a surface resistivity of 1.0xc3x97108 xcexa9 or below and a volume resistivity of 1.0xc3x97108 xcexa9cm or above.
The present invention also provides a fixing assembly comprising a fixing member and a pressure member coming into pressure contact with the fixing member to form a fixing nip, wherein;
a recording material holding an unfixed toner image thereon is passed through the fixing nip so as to fix the unfixed toner image to the recording material to form a fixed imaged on the recording material; and
the fixing member comprises a conductive layer and a releasing layer formed on the conductive layer, wherein;
in the releasing layer, any one or both of medium-resistance particles and medium-resistance whiskers are dispersed; and
the releasing layer has a surface resistivity of 1.0xc3x97108 xcexa9 or below and a volume resistivity of 1.0xc3x97108 xcexa9cm or above.
The present invention still also provides an image-forming apparatus comprising:
an image-bearing member for holding thereon an electrostatic latent image;
a charging means for charging the surface of the image-bearing member electrostatically;
an electrostatic latent image forming means for forming an electrostatic latent image on the surface of the image-bearing member thus charged;
a developing means having a developer and by which the electrostatic latent image formed on the image-bearing member is developed to form a toner image;
a transfer means for transferring the toner image onto a recording material; and
a fixing assembly by means of which the toner image transferred onto the recording material and standing unfixed is fixed to form a fixed image;
wherein;
the fixing assembly comprises a fixing member and a pressure member coming into pressure contact with the fixing member to form a fixing nip, wherein;
a recording material holding an unfixed toner image thereon is passed through the fixing nip so as to fix the unfixed toner image to the recording material to form a fixed imaged on the recording material; and
the fixing member comprises a conductive layer and a releasing layer formed on the conductive layer, wherein;
in the releasing layer, any one or both of medium-resistance particles and medium-resistance whiskers are dispersed; and
the releasing layer has a surface resistivity of 1.0xc3x97108 xcexa9 or below and a volume resistivity of 1.0xc3x97108 xcexa9cm or above.